1. Technical Field
This invention relates to a method of finding the best possible equalization of an electric input signal which was derived from an optical signal transmitted over an optical fiber and which is distorted due to interference in the optical signal, particularly as a result of polarization mode dispersion (PMD), and to an electrical equalizing facility for such an electric input signal comprising an electronic equalizer.
2. Discussion of Related Art
Such an electrical equalizing facility is described, for example, in an article by J. Winters et al, "Electrical Signal Processing Techniques in Long-Haul Fiber-Optic Systems", IEEE Transactions on Communications, Vol. 38, No. 9, 1990, pages 1439 to 1453.
Polarization mode dispersion (PMD), which occurs in single-mode fibers, manifests itself in a two-way propagation of an optical signal. If a fiber exhibits birefringence, i.e., different propagation conditions for the two orthogonal directions of polarization (principal axes), the fundamental mode is split into two modes polarized in mutually perpendicular planes. Along these two principal axes of the fiber, the optical signal propagates at different group velocities, i.e., a "fast" and a "slow" signal component results. The PMD can be sufficiently characterized as a function of two quantities, e.g., by the time difference AT between the "fast" and "slow" signal components and by the relative power .gamma. in one of the principal axes. In connection with very-high-bit-rate optical transmission on optical fibers (&gt;2.5 Gb/s), PMD may occur as a property which limits the transmission rate (maximum transmission distance).
From an article by T. Takahashi et al, "Automatic compensation technique for timewise fluctuating polarisation mode dispersion in in-line amplifier systems", Electronics Letters, Vol. 30, No. 4, 1994, pages 348/349, it is known to compensate for PMD-induced signal distortions in the optical domain by adaptively inserting an opposite birefringence in the optical signal path, thereby reducing the distortion. This is accomplished by inserting a polarization controller and an optical delay line in the optical signal path in front of the optical receiver.
PMD compensation in the electrical domain is known from the above-mentioned article by J. Winters et al. To compensate for linear distortion, a linear electronic equalizer is provided in an optical receiver after the photodiode. For the electronic equalizer, an electronic filter with N taps is used as a branching delay line. Both transversal filters and decision feedback equalizers (DFEs) are proposed whose parameters are caused to track the time-varying PMD.
Since the PMD is not constant over time, in the optical domain, the delay line would have to be adapted to the time-varying PMD, which would only be possible with a large amount of circuitry. In the electrical domain, the problem is that with a transversal filter, for example, compensation is not possible for .gamma.=0.5, where .gamma. is the relative ratio of the powers in the two states of polarization. A DFE, in turn, does not permit equalization at small y. Since in an electronic equalizer many equalization parameters have to be set independently of each other, finding the best possible equalization of an electric input signal distorted as a result of PMD requires a complex and time-consuming optimization algorithm.